Diseases of the liver are a major world wide health problem, yet current diagnostic tests have shortcoming in assessing the functional hepatocyte mass of an individual. In our Phase I research, we developed an MR contrast agent which exhibited hepatocyte specific (liver specific) biodistribution, by virtue of its recognition by asialoglycoprotein receptor. The hepatocyte specific MR contrast agent is a functionalized (or coated) superparamagnetic iron oxide colloid. Using an animal model of human liver pathology, phase II research will pursue two major goals. First, it will establish that the clearance of the agent, as determined MR imaging, provides a measure of functional hepatocyte mass. And second, it will establish that the intrahepatic distribution of the agent provides information on the functional anatomy of the liver. Surface functionalized superparamagnetic iron oxide colloids may prove to be a broad class of MR contrast agents, targeted to a variety of receptors (and tissues) in vivo.